Method and Apparatus for Initiating Communications on a Shared Channel in a Mobile Communications System

1. A method for performing random access in a user equipment (UE) of a mobile communication system, the method comprising: receiving system information indicating a group of identification(ID)s; selecting an first ID from among the group of the IDs; transmitting a first uplink signal corresponding to the selected first ID for random access to a Node B; after the transmitting of the first uplink signal, waiting for a predetermined delay duration without checking a downlink channel; after the waiting for the predetermined delay duration, checking the downlink channel during a valid period; determining whether a downlink signal responding to the first uplink signal is received in the valid period, the downlink signal comprising a second ID and an UE-ID; and transmitting a second uplink signal using the UE-ID, if the downlink signal is received during the valid period and the second ID is equal to the first ID, wherein the valid period starts when the predetermined delay duration starting from transmission of the first uplink signal has terminated.

2. The method of claim 1 , further comprising: acquiring information indicative of the valid period from the system information broadcasted in a current cell.

3. The method of claim 1 , wherein the second ID is one a plurality of identifiers in an identifier pool of a current cell.

4. A user equipment (UE) for performing random access in a mobile communication system, the UE comprising: a receiver configured to receive system information indicating a group of identification(ID)s; a signal generator configured to: select an first ID from among the group of the IDs, and generate a first uplink signal corresponding to the selected first ID for random access; a downlink signal processor configured to: after transmitting the first uplink signal, wait for a predetermined delay duration without checking a downlink channel, after waiting for the predetermined delay duration, check the downlink channel during a valid period, and determine whether a downlink signal responding to the first uplink signal is received in the valid period, the downlink signal comprising a second ID and an UE-ID; and a transceiver configured to: transmit the generated first uplink signal to a Node B, and transmit a second uplink signal using the UE-ID, if the downlink signal is received during the valid period and the second ID is equal to the first ID, wherein the valid period starts when the predetermined delay duration starting from transmission of the first uplink signal has terminated.

5. The UE of claim 4 , further comprising: acquiring information indicative of the valid period from the system information broadcasted in a current cell.

6. The UE of claim 4 , wherein the second ID is one a plurality of identifiers in an identifier pool of a current cell.

7. A method for performing random access in a Node B of a mobile communication system, the method comprising: transmitting system information indicating a group of identification(ID)s; receiving a first uplink signal corresponding to a first ID selected from among the group of the IDs for random access from a user equipment (UE); after the receiving of the first uplink signal, waiting for a predetermined delay duration without transmitting a downlink signal responding to the first uplink signal; after the waiting for the predetermined delay duration, transmitting a downlink signal responding to the first uplink signal in a valid period, the downlink signal comprising a second ID and an UE-ID; and receiving a second uplink signal using the UE-ID, if the second ID is equal to the first ID, wherein the valid period starts, when the predetermined delay duration starting from transmission of the first uplink signal has terminated, and wherein data is transmitted and received after the transmitting of the downlink signal during the valid period.

8. The method of claim 7 , wherein the system information comprises information indicative of the valid period.

9. The method of claim 7 , wherein the second ID is one a plurality of identifiers in an identifier pool of a current cell.

10. A Node B for performing random access in a mobile communication system, the Node B comprising: a transceiver configured to: transmit system information indicating a group of identification(ID)s, receive a first uplink signal corresponding to a first ID selected from among the group of the IDs for random access from a user equipment (UE), after receiving the first uplink signal, wait for a predetermined delay duration without transmitting a downlink signal responding to the first uplink signal, after waiting for the predetermined delay duration, transmit a downlink signal responding to the first uplink signal in a valid period, the downlink signal comprising a second ID and an UE-ID, and receive a second uplink signal using the UE-ID, if the second ID is equal to the first ID; and a controller configured to control the transceiver to transmit the downlink signal in the valid period, wherein the valid period starts when the predetermined delay duration starting from transmission of the first uplink signal has terminated, and wherein data is transmitted and received after transmitting the downlink signal during the valid period.

11. The Node B of claim 10 , wherein the system information comprises information indicative of the valid period.

12. The Node B of claim 10 , wherein the second ID is one a plurality of identifiers in an identifier pool of a current cell.